The US Centers for Disease Control estimate that over 30,000 children born this year will eventually be diagnosed with some form of autism, potentially putting them and their parents through a demanding ordeal. In its most extreme forms, autism is a debilitating condition that impairs or completely inhibits language development, and causes uncontrolled outbursts and repetitive behaviors like rocking, hand flapping, or head rolling. Other children who are diagnosed with autism may have trouble communicating and reading emotions in others, but can grow up to be fully independent and have successful, challenging careers. Because there is such variation in the symptoms of autism, most doctors now talk about autism as a spectrum of disorders: ASD.

Although study results vary, research has found that as many as one percent of all US children have an ASD. How are all those children stricken? What actually causes autism? A quick Web search for “autism” reveals an array of answers, only some of which are supported by scientific research. An advertisement at the top of my search results on Google led me to an official-looking website arguing forcefully that autism is caused by minute quantities of thimerosal found in some vaccines, despite the fact that no reputable study has ever shown a causal link between vaccines and autism. The site contended that studies disputing the relationship between autism and vaccines containing thimerosal were poorly constructed. It also cited Andrew Wakefield’s 1998 study on the correlation between the measles, mumps, and rubella (MMR) vaccine and autism. But Wakefield’s study involved just 12 children, was later retracted, was complicated by conflicts of interest, and may have been fraudulent. Yet after all that, the site offered no hard evidence supporting a causal link between vaccinations and autism! Even if the Wakefield study wasn’t seriously flawed, it could never have shown that vaccines cause autism. So what do larger, rigorously constructed studies have to say about the roots of ASD?

University of Michigan child psychologist Nestor Lopez Duran has been tracking the research on causes and treatment of autism for years, and last week he discussed a massive study of a population of 70,000 children born between 1994 and 1999, released this month in Pediatrics. The study confirmed what had already been found dozens of times before, that there is no link between vaccines and autism.

The team of researchers in this latest study identified 256 children with autism from the larger study group, and matched each child with autism with another three normally-developing children of similar age and background. They then took a careful look at the vaccination history of all the children, and found that regardless of when vaccines were given and in what quantity, there was no association between thimerosal in the vaccines and an ASD diagnosis. Thus over a decade of work showing no relationship between vaccines and autism was confirmed, once again.

But if vaccines aren’t to blame for autism, what is?

While the focus of the public debate, fueled by celebrities with axes to grind, has been on the link to vaccines, scientists have been taking a close look at genetics. As cancer surgeon and biologist David Gorski wrote in June, the genetics of complex conditions like cancer and autism are, well, complex. Rather than a single genetic anomaly causing a problem, it seems more likely that a large set of factors would combine to cause ASD. Gorski (and computational biologist Grant Jacobs) explained a study by a vast team of researchers led by Dalila Pinto and published in Nature in June. The researchers analyzed the genomes of over 3,000 children, 1,275 of whom had been diagnosed with an ASD. They found that the genomes of kids with ASD showed significant differences from the developmentally normal kids. They also developed a plausible model of how ASD could arise in these affected children. Although the details will probably take many years to work out, children with ASD had more copy number variations (CNVs)—duplicates or missing copies of portions of the genome—than others. The CNVs occurred in portions of the genome associated with cell migration and adhesion, which Gorski says may influence the development of neural connections in the brain. It’s plausible that these variations could cause changes that might lead to autism, but much more work needs to be done before we know for sure.

Certainly the research pointing to a genetic cause for ASD is much stronger than any link between thimerosal in vaccines and the disorder. Yet millions of parents are swayed by celebrities who advocate against vaccination and hucksters promoting miracle cures for the “toxins” that they claim are found in vaccines and other innocuous products. Why?

Some of the answer may come from cognitive biases. Consider this contrived-yet-classic example:

You work in a switch house for a commuter rail company. A train car is rushing out of control towards five workers on the tracks. You can let the workers die, or press a button, switching the train to another track, where just one worker stands in the train’s path. In both cases, the workers face certain death. Would you press the button?

Now suppose you’re walking across a bridge over the tracks. There’s no switch, but the same car is hurtling towards the same five workers. You can save them by pushing a large man off the bridge; his body will stop the train and save the workers, but he would die in the process. Would you do it?

Michelle Dawson explains why we should be skeptical about so-called autism cures.

Most people in the Western world would say yes to the first question, and no to the second. Why? As Sam Harris argues in his new book The Moral Landscape, we have a bias against sins of commission rather than sins of omission. In the first scenario, you’re less directly implicated in the death of the worker; all you did was press a button. In the second scenario, you would have had to effectively commit murder to prevent the workers’ deaths—even though the end result in both cases is equivalent, somehow playing a more active, physical role just seems worse.

Biases like this can go a long way toward explaining why millions of people are so easily convinced that the measles vaccine, despite overwhelming scientific evidence to the contrary, causes autism. Parents hear that some children develop autism shortly after getting government-mandated vaccines, and they decide that vaccines must be bad, despite the fact that they prevent deadly disease. Here they see the government or meddling doctors causing autism—a sin of commission. But parents who don’t get their children vaccinated and end up causing a measles outbreak are only committing the lesser sin of omission.

Humans are also naturally biased to favor their own children over others’ kids, and to prioritize present dangers (like the growing autism crisis) over remote ones (like the dim memory of measles). Other biases may be in effect as well. Powerful biases such as these are difficult to overcome, but perhaps as we begin to see more and more potentially deadly outbreaks of preventable diseases, public opinion will change. Our attention to the question of autism might then be redirected towards research on effective treatments—but sadly, only at the cost of serious illness in children who might have been protected by vaccines.

Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »